1. Field of the Invention
The invention concerns an eyeglass frame comprising at least one variable length member and is more particularly concerned with an improvement exploiting the properties of shape memory alloys and in particular making it possible, by means of an unusually simple arrangement, to adjust easily and accurately the length of the temples according to the shape of the user's head, the adjustment being retained with a greater force than in prior art devices. The same principle applies to adjusting the bridge of the eyeglass frame.
2. Description of the Prior Art
There are known eyeglass frames with adjustable temples comprising, for example, two parts adapted to slide one inside the other over a certain distance and to be immobilized relative to each other in a selected position using some form of gripping means. Thus French patent No. 679 208 discloses a system of this kind in which a cap is screwed onto one end of one part of each temple to clamp a wedging ring deformation of which immobilizes the other part of the temple.
Such devices are costly and are not entirely satisfactory given that the clamping means may be loosened, whereby the adjustment is lost.
The invention proposes a new and simpler arrangement that is truly unreleasable under normal conditions of use, specifically over a wide range of ambient temperatures. The invention constitutes an application of shape memory alloys that have been commercially available for a number of years. An overview of the properties of such alloys is given, for example, in the journal "Sciences et Techniques" of June 1985, No 16. It is known that a member formed from an alloy of this kind has the property of resuming a given shape as the result of a variation in temperature or the application of stresses. The dimensions of an object may be varied to a significant degree (between 5 and 8%) in response to a relatively modest increase or decrease in temperature, before resuming the "stored" initial state when the object is returned to its original temperature.
This kind of property can be exploited, depending on the type of alloy and how it is used, either by heating or by cooling and in order to produce either an expansion or a shrinkage; it is exploited to obtain what might be termed a binding action that is easy to implement, notably in an optometrist's store and preferably using a cooling device.
PCT application No. WO 85/02688 proposes eyeglass frames incorporating such alloys. The invention proposes a solution of the same type that is less costly.